In many of the posts on NRF there is a wide range of opinions and some of us are certain our perception is the correct one. Lately in trying to do a lens test, I have had enormous difficulties getting accurate focus. Mainly this is due to my not having the subject properly lighted. In assessing my errors in getting the lens test completed in a manner useful to those on NRF, I became of my "assumptions" and limits to my perceptions.
Well, in this spirit, I would offer to those who are into illusions, the opportunity to see how easily fooled we are. This is from a beautiful website, Astronomy Picture of the Day